1. Field of the Invention
The present invention relates to a toner which is useful for developing electrostatic images and is suitable for fixation by hot-pressing. The present invention also relates to a process for producing the toner.
2. Related Background Art
Electrophotographic methods have been known as shown in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748, and so forth. Generally in electrophotography by using a photoconductive material, images are copied or printed by forming an electrostatic image on a photosensitive member, developing the electrostatic image with a toner to form a toner image, transferring the toner image onto a toner image-receiving medium (transfer medium) such as a paper sheet, and fixing the toner image by heating, pressing, hot-pressing, solvent vapor exposure, or a like method.
Various methods have been disclosed for developing and fixing electrostatic images with a toner, and a suitable method is selected for the respective image forming process. Conventionally, the toner used for the above method is produced generally by a process comprising melt-blending a colorant composed of a dye and/or a pigment into a thermoplastic resin to form a uniform dispersion, pulverizing the blended matter, and classifying the pulverized matter to separate toner particles having an intended particle size.
Such a production process produces a toner of sufficiently high quality under some limitations. For example, the colorant-containing resin composition should be brittle to be pulverizable economically by a pulverizing apparatus. However, the colorant-containing resin composition which has been made brittle tends disadvantageously to have broader particle size distribution after a high-speed pulverization to contain relatively larger particles. Moreover, such a brittle toner material tends to be further crushed or pulverized during image development. In such a production process, a fine solid particulate material like a colorant cannot readily be dispersed sufficiently uniformly in a resin. The insufficient dispersion can cause increased fogging, lower image density, lower toner color mixing characteristics, or lower transparency. A colorant which is uncovered on the broken surface of the toner may cause variation of development characteristics of the toner.
To solve the problems involved in the toner produced by the aforementioned pulverization, suspension polymerization for producing the toner is disclosed in Japanese Patent Publication Nos. 36-10231, 42-10799, and 51-14895. In the suspension polymerization, a polymerizable monomer, a colorant, and a polymerization initiator, and optionally a crosslinking agent, a charge-controlling agent, and other additives are mixed to form a monomer composition of a uniform solution or dispersion, and the polymerizable monomer is polymerized in an aqueous medium containing a dispersion stabilizer with agitation by a suitable mixer to form toner particles having a desired particle diameter.
Therefore, the toner produced by the suspension polymerization need not be pulverized, so that the toner material is not required to be brittle or may be used a soft material. Further, the colorant does not become uncovered on the toner particle surface because of the absence of the pulverization process, thereby uniform frictional electrification characteristics of the toner are achievable. Furthermore, the particle classification operation can be omitted to provide cost reduction effects such as saving of energy, shortening of production time, and improvement of process yield.
However, when the toner produced by such a process is further pulverized, it tends to have the colorant uncovered on the particle surface thereof which causes decrease of uniformity of the electrification and variation of the developing characteristics. This phenomenon is significant particularly when the copying or printing is continued under high humidity conditions.
For uniform electrification of the toner, methods have been disclosed, for example, in Japanese Patent Application Laid-Open Nos. 62-73277 and 3-35660, in which the surface layer of toner particles is covered with a resin. In these methods, the absolute value of the electric charge becomes smaller disadvantageously, because only a small amount of charge-controlling agent can be incorporated, although the thick coating layer counteracts the above adverse effects of the uncovered colorant.
To solve the above problem, multi-layer coating of the toner particles is disclosed in Japanese Patent Application Laid-Open Nos. 64-62666 and 64-63035, and Japanese Patent Publication No. 58-57105. However, the production process therefor is complicated and unduly costly.
To overcome such disadvantages, a charge-controlling agent is deposited on the toner particle surface as disclosed in Japanese Patent Application Laid-Open Nos. 61-273558 and 5-134437. This method, however, causes a problem in the release of the charge-controlling agent from the toner surface which reduces the toner durability in copying or printing a plurality of sheets.
In recent years, digital full-color copying machines and printers have been commercialized which are capable of producing images of high quality with high resolution, high gradation, high color reproducibility without color irregularity. In a digital full-color copying machine or printer, the color of an original color image is separated into elemental colors by use of filters of B (blue), G (green), and R (red). Electrostatic images constituted of dots of a diameter ranging from 20 to 70 .mu.m corresponding to the original image are developed with toners of Y (yellow), M (magenta), C (cyan), and Bk (black) according to the subtractive color mixing principle. In color copying or printing, the particles of the toner are required to be finer in size to obtain fine dots for high image quality since the toners are transferred in larger amounts than in monochromatic copying from the photosensitive members onto the toner image-receiving medium.
Improvement of the low-temperature fixabtlity of the toner is important in consideration of future increases in the printing speed and of future progress in full-color copying. From this standpoint, the toner produced by polymerization is preferred since it can be produced relatively easily in fine particle size with sharp particle size distribution.
The colors of the respective color toners used in a full-color copying machine or a full-color printer should be miscible sufficiently with each other in the fixation step. In full-color copying or printing, color reproducibility is important, and transparency of an overhead projector (abbreviated as OHP) image is required. Further, the color toners are desirably composed of a lower molecular weight resin to be more readily fusible than the black toner.
For a black toner, a relatively highly crystalline wax such as polyethylene waxes and polypropylene waxes is used as a releasing agent in order to improve the high-temperature offset resistance at the image fixation step. In full-color image formation for OHP, however, the high crystallinity of the wax lowers the transparency of the formed image.
Therefore, a releasing agent is not usually incorporated into color toners, but an offset-preventing agent such as a silicone oil is applied onto a hot-fixing roller to improve the high-temperature offset resistance. However, the excess silicone oil adhering onto a toner image-receiving medium may undesirably provide the user with an unpleasant feeling on handling the medium after the fixation. To solve this problem, an oil-free fixing toner was investigated which contains a large amount of low-temperature softening substance in a toner particle. A toner is demanded which is superior in low-temperature fixability and transparency and has high-temperature offset resistance.
To solve these problems, Japanese Patent Application Laid-Open No. 1-230073 discloses a color image fixing process employing a polymerized toner containing a low-temperature softening substance having releasability. This toner is liable to deteriorate in toner developing properties during many sheets of copying or printing, which is considered to be ascribable to exudation of the low-temperature softening substance to the toner particle surface.
Japanese Patent Application Laid-Open No. 61-35457 describes addition of a polar polymer or copolymer to the polymerizable monomer composition to prevent exposure of the colorant or the exudation of the low-temperature softening substance to the toner particle surface. Japanese Patent Application Laid-Open No. 6-317925 discloses formation of a hydrophilic outer shell on the surface of the toner particles for the same purpose. However, the hydrophilicity of the shell-forming material will impair the development characteristics of the toner under high humidity conditions. Further, in this method, the glass transition temperature of the core resin is set at a temperature ranging from 10.degree. C. to 50.degree. C. in order to reduce the interruption of fixing by the shell material, which tends to cause sticking of the toner image-receiving medium onto the fixing roller in toner image fixation.
Accordingly, a toner, in particular a color toner, is desired which is produced by polymerization and yet does not involve the aforementioned problems in development characteristics and fixation characteristics.